An arc furnace is an electrically operated furnace used for melting metal and/or for cleaning slag. The operation of the furnace is based on an electric arc that burns either between separate electrodes, or between electrodes and the material to be melted. The furnace can be operated either by alternating or direct current. Heat is created in the electric arc, and also in the material to be melted, in case the electric arc burns between the material and the electrodes. Power is conducted to vertical electrodes that are located symmetrically in a triangle with respect to the midpoint of the furnace. The assembly depth of the electrodes in the furnace is adjusted from time to time, when they are worn at the tips.
The electrodes extend into the furnace via through holes located in the furnace ceiling. The diameter of a through hole is larger than the diameter of an electrode, in order to ensure free motion of the electrode, and in order to avoid contact between the electrode and the ceiling. The gap left between the electrode and the ceiling aperture must be sealed by a sealing device in order to prevent the access of gases from inside the furnace through the aperture to the atmosphere, and on the other hand in order to prevent the access of air from the atmosphere to the furnace.
In the prior art there are known sealing devices for sealing the gap left between the electrode and the ceiling aperture by mechanical sealings, for instance by graphite rings, graphite rope seals etc. that are hydraulically pressed against the electrode. Various mechanical sealing arrangements are known for example from the publications FI 81197, FI 64458, DE 1540876, and SE 445744. The hydraulic medium used for creating hydraulic compression between the graphite rings and the rope sealings is water.
A drawback with known sealing devices is the use of water as the pressurizing medium in connection with sealing, because in a damage situation, water may accidentally get into the furnace. When water is introduced into the furnace atmosphere with a high temperature, a dangerous water-gas explosion may occur.